1. Field of the Invention
This invention relates generally to an integrated microwave oscillator with high density in which a 3-terminal active element is used as an oscillating element, and is directed more particularly to an integrated microwave oscillator which is simple in construction but can operable stably even for any temperature variation.
2. Description of the Prior Art
In the art there has been proposed a microwave oscillator which utilizes a 2-terminal negative resistance element or 3-terminal active element, for example, gallium-arsenide field effect transistor (which will be hereinafter referred to simply as G.sub.a A.sub.s -FET) as an oscillating element. A design of the oscillator which uses the G.sub.a A.sub.s -FET is disclosed in, for example, IEEE MTT-23, No. 8, pages 661 to 667 Design and Performance of X-Band Oscillators with GaAs Schottky-Gate Field-Effect Transistors which was issued in August 1975 by Maeda, et al. The advantages of the oscillator using a G.sub.a A.sub.s -FET are that it is low in noises, high in efficiency and low in bias voltage as compared with an oscillator which uses a Gunn diode or IMPATT diode.
In a microwave oscillating circuit which uses a 3-terminal active element as its oscillating element there are two kinds, namely a series feedback type oscillating circuit and a parallel feedback type oscillating circuit whose equivalent circuits are illustrated in FIGS. 1 and 2, respectively.
In FIG. 1 which shows the series feedback type oscillating circuit, numeral 1 designates a G.sub.a A.sub.s -FET or 3-terminal active element as an oscillating element, 2, 3 and 4 designate its gate, drain and source terminals, respectively. Numeral 5 designates a series feedback circuit as a positive feedback circuit which is formed of inductive or capacitive elements 6 and 7. In this case, it is selected that if one of these elements 6 and 7 is inductive, the other element is capacitive. The gate terminal 2 is connected to one end of a capacitive or inductive element 6 of the positive feedback circuit 5, the source terminal 4 is similarly connected to one end of the inductive or capacitive element 7 of positive feedback circuit 5, and the drain terminal 3 is connected to one end of a load impedance element 8. The other ends of these elements 6, 7 and 8 are connected common.
In FIG. 2 which shows a parallel feedback type circuit, numeral 1 similarly designates a G.sub.a A.sub.s -FET as a 3-terminal active element which is used as an oscillating element, and 9 designates a parallel feedback circuit as a positive feedback circuit. The parallel feedback circuit 9 is formed of inductive or capacitive elements 10 and 11 which are selected such that when one of them is inductive the other is capacitive. The inductive or capacitive element 10 is connected between gate terminal 2 and source terminal 4 of G.sub.a A.sub.s -FET 1 and the capacitive or inductive element 11 is connected between gate terminal 2 and drain terminal 3, respectively. Further, a load admittance element 12 is connected between drain terminal 3 and source terminal of G.sub.a A.sub.s -FET 1.
If the positive feedback circuits 5 and 9 of oscillating circuits shown in FIGS. 1 and 2 are made by using a line such as a micro-strip line and so on, they can be made as an integrated microwave oscillator.
The practical construction of the series feedback type oscillator shown in FIG. 1 will be now described with reference to FIG. 3. In FIG. 3, numeral 13 designates a substrate of a microwave integrated circuit which consists of a dielectric made of, for example, alumina Al.sub.2 O.sub.3, a conductive layer formed uniformly on its rear surface and a conductive layer of a desired pattern on the front surface and a conductive layer of a desired pattern on the front surface of the dielectric. The G.sub.a A.sub.s -FET 1 is provided on the substrate 13 as a 3-terminal active element for oscillation. The gate terminal 2 of G.sub.a A.sub.s -FET 1 is connected to a micro-strip line 15 whose tip end is opened and which forms the element 6 shown in FIG. 1 and the source terminal 4 of G.sub.a A.sub.s -FET 1 is connected to a micro-strip line 16 which forms the element 7 and whose tip end is short-circuited. The tip end of micro-strip line 16 is short-circuited by connecting the tip end to an earth conductor 17. The drain terminal 3 of G.sub.a A.sub.s -FET 1 is connected to a micro-strip line 18 which is, in turn, connected through a gap capacitor 19 of choking a DC current to a high frequency output terminal 20.
On the substrate 13 there are provided high impedance lead wires 21 and 22 whose tip ends are connected to micro-strip line 15, to which the gate terminal 2 is connected, and to micro-strip line 18 to which the drain terminal 3 is connected, respectively. Positive and negative DC biases are applied through the lead wires 21 and 22 to the gate and drain of G.sub.a A.sub.s -FET 1 from the outside.
The micro-strip lines 15, 16 and 18 of the above circuit can become capacitive elements or inductive elements in accordance with the relation of their lengths to the wavelength of a microwave. To this end, the lengths of micro-strip lines 15, 16 and 18 are selected suitably.
Since the equivalent circuit of the series feedback oscillator of FIG. 3 can be shown in FIG. 1 as described previously, its operation will be described with reference to FIG. 1. If it is assumed that the output impedance seen to the active element 1 including the positive feedback circuit 5 from the both ends of load impedance element 8 is taken as Z.sub.out and the impedance of load impedance element 8 as Z.sub.L, the oscillation frequency f.sub.o of the oscillator is determined as a frequency which will satisfy the frequency oscillaton condition, i.e. the following equation (1). EQU I.sub.m (Z.sub.out)+I.sub.m (Z.sub.L)=0 (1)
where I.sub.m (Z.sub.out) and I.sub.m (Z.sub.L) represent the imaginary number portions of Z.sub.out and Z.sub.L, respectively.
In the case of the parallel feedback type oscillator which is not shown but whose equivalent circuit is shown in FIG. 2, its oscillation frequency f.sub.o is similarly determined as a frequency which will satisfy the following equation (2). EQU I.sub.m (Y.sub.out)+I.sub.m (Y.sub.L)=0 (2)
if the function of the left side of equation (1) or (2) is taken as F, this is a function whose variables are the angular frequency .omega. and all circuit parameters forming the oscillator such as the active element, passive elements, for example, micro-strip lines and so on. While, the respective circuit parameters vary in accordance with variation of temperature T, so that the circuit parameters are the function of the temperature T. Further, the function F is a function of the angular frequency .omega. and temperature T. Therefore, the function F can be expressed as F (.omega., T). At this time, the oscillation condition is expressed by the following equation (3) EQU F(.omega., T)=0 (3)
accordingly, the change of angular frequency .omega. of the oscillation frequency for temperature T or .differential..omega./.differential.T can be expressed by the following equation (4). ##EQU1## From the equation (4) it will be understood that in order to make the temperature change .differential..omega./.differential.T of angular frequency .omega. of the oscillation frequency small, it is sufficient to decrease .differential.F/.differential.T or to increase .differential.F/.differential..omega..
As shown in FIG. 3, with the prior art integrated microwave oscillator, its feedback circuit is formed by making the rip ends of lines such as the strip lines open or short-circuited. Therefore, it can not be possible to select the value of .differential.F/.differential..omega. in the equation (4) so great and hence the temperature change .differential..omega./.differential.T of the oscillation frequency can not be made so small.